<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    html,
    body {
      height: 100%;
      padding: 0;
      margin: 0;
    }

    .outer {
      height: 100%;
      padding: 100px 0 0;
      box-sizing: border-box;
    }

    .A {
      margin: -100px 0 0;
      background: #BBE8F2;
    }

    .B {
      height: 100%;
      background: #D9C666;
    }
  </style>
</head>

<body>
  <div class="outer">
    <div class="A"> 头部DIV </div>
    <div class="B">下部DIV </div>
  </div>
</body>

</html>